1. Field of the Invention
This invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material comprising a support and a thermosensitive coloring layer formed on the support, which thermosensitive coloring layer comprises at least a first thermosensitive coloring layer which comprises a leuco dye having absorption intensity in the near infrared region, and a second thermosensitive coloring layer which comprises a leuco dye having absorption intensity in the visual spectrum.
2. Discussion of Background
Conventionally, there is known a thermosensitive recording material, in which a thermosensitive coloring layer mainly comprising a thermosensitive coloring composition is provided on a support such as a sheet of paper and synthetic paper, or a plastic film. In such a recording material colored images are obtained by application of heat to the recording material using a thermal head, thermal pen, laser beam, and the like.
This kind of recording material has been widely utilized in various fields because of the following advantages over other conventional recording materials:
(1) image recording can be speedily performed, using a comparatively simple device without complicated steps for development and image fixing;
(2) the thermosensitive recording material can be produced and used without generating noise and causing environmental pollution; and
(3) the manufacturing cost of the thermosensitive recording material is low.
Recently, an optical character reader and a bar-code reader have been developed and utilized increasingly. As a light source in such character readers, a light emitting diode or a semiconductor laser beam having a wavelength of 700 nm or more is conventionally used. However, leuco dyes such as fluoran-type leuco dyes and triphenylmethane-type leuco dyes for use in the conventional thermosensitive recording materials can hardly absorb the near infrared rays having a wavelength of 700 nm or more. Therefore the images formed on the conventional thermosensitive recording materials can be read by neither the optical character reader nor the bar-code reader.
Several proposals relating to the leuco dyes having absorption intensity in the near infrared region have been made, for example, as disclosed in Japanese Patent Publication 58-5940, and Japanese Laid-Open Patent Applications 59-199757, 60-230890, 62-106964, 62-243653, 62-257970, and 63-37158.
However, these leuco dyes having absorption intensity in the near infrared region have the shortcomings that the leuco dyes are colored or a color tone of the obtained images is not black.
Moreover, the above proposals also disclose that the conventionally used leuco dyes having absorption intensity in the wavelength of less than 700 nm, such as fluoran-type leuco dyes and triphenylmethane-type leuco dyes, are used in combination with the above-mentioned leuco dyes having absorption intensity in the near infrared region. In such a case, however, inconsistency of coloring sensitivity between in the visible spectrum and in the near infrared region is caused. In addition, the deterioration in the heat resistance of the thermosensitive recording materials induces the fogging of the background thereof.